The present invention relates to a high pressure discharge lamp in which the hot restriking voltage is reduced, and the hot restriking time is decreased compared to prior art lamps of the same type.
In general, in a high pressure discharge lamp such as a high pressure mercury lamp, a metal halide lamp or a high pressure sodium lamp, mercury vapor pressure in the arc tubes of the lamps reaches several atmospheres during operation of the lamps. In these types of lamps, if the lamps are turned off because of a temporary drop of the supply voltage, the lamps cannot be started again immediately when the supply voltage is restored to its normal value. That is, it is impossible to start the lamps again until the temperature of the arc tubes drops to a sufficiently low level that the mercury vapor pressure in the arc tubes is low enough for the discharge to be started. The hot restriking time is typically three to five minutes in a high pressure mercury lamp, eight to fifteen minutes in a metal halide lamp, and two to fifteen minutes in a high pressure sodium lamp. The use and operation of high pressure discharge lamps requiring such a long restriking time is troublesome.
In order to reduce the long restriking time and to restart the lamp substantially instantly, a method has been employed in which the lamp is forcibly restarted by applying a high voltage of 10 to 35 kv between the electrodes. In this method, the applied voltage must be very high, andd therefore the lamp and the lighting device must be specially constructed and consequently large in size and they have a high manufacturing cost. In addition, because of the use of high voltage, installation of the lighting device is unavoidably troublesome.
A conventional 400 w high pressure mercury lamp will be described with reference to FIG. 1 in detail. Main electrodes 2 and 3 are provided at both ends of a quartz arc tube 1 and an auxiliary electrode 4 is provided adjacent to the main electrode 2. Necessary amounts of mercury and argon are sealed in the arc tube 1. The main electrodes 2 and 3 are connected respectively through leads 8 and 9 to a lamp operating apparatus 12 with a ballast. The auxiliary electrode 4 is connected through a lead 7 and a starting resistor 6 having a resistance of several tens of kilo-ohms (K.OMEGA.) to the lead 9. In FIG. 1, reference numeral 11 designates a transparent outer bulb enclosing the tube 1 and reference numeral 13 designates a power source.
When the high pressure mercury lamp thus constructed is connected through the lamp operating apparatus 12 to the power source 13 to apply a voltage to the lamp, at first a glow discharge occurs between the main electrode 2 and the auxiliary electrode 4 and current limited by the starting resistor 6 flows. As a result of this auxiliary discharge, a discharge is quickly started between the main electrodes 2 and 3. The current between the main electrodes 2 and 3, the lamp current, is controlled by the lamp operating apparatus 12. The lamp reaches a stable operating state about five minutes after the discharge has been started. The mercury vapor pressure in the arc tube 1 reaches about five atmospheres during operation and a stable mercury discharge is maintained between the main electrodes 2 and 3.
If the high pressure mercury lamp during operation is turned off, for instance, by a temporary drop of the supply voltage, even if the supply voltage is quickly restored to its normal value, the lamp cannot restart the discharge immediately because the mercury vapor pressure is high, typically several atmospheres.
In general, the hot restriking time for a high pressure mercury lamp is three to five minutes as described above. During the time, the temperature of the arc tube decreases, and accordingly the mercury vapor pressure therein decreases. When the temperature of the arc tube drops to about 150.degree. C. from about 600.degree. C. (which is the temperature of the arc tube in operation), the lamp can be restarted. At about 150.degree. C., the mercury vapor pressure in the arc tube is about 3 Torr.
The relation between the time elapsed after the lamp has been turned off and the starting voltage needed to restart the lamp is as shown in FIG. 2. The starting voltage increases with time and reaches its maximum value, higher than 8 kv, in about one minute. In order to restart the discharge at that time, a high voltage higher than 8 kv must be applied to the lamp. As time elapses further, the starting voltage gradually decreases. In about four minutes, the discharge can be started with the ordinary line voltage (for instance, 200 v).
As is apparent from the above description, in order to restart the lamp at any time after it has been turned off, it is necessary for the lamp operating apparatus to be capable of supplying a voltage higher than 8 kv. The voltage applied depends on the type of lamp and output power. However, in practice, the high voltage is in a range from 10 kv to 35 kv.
The inventors have investigated the relation between the mercury vapor pressure in the arc tube and the lamp starting voltage, and have found the relation to follow the plot in FIG. 3. When the mercury vapor pressure is lower than about 10.sup.-1 Torr corresponding to a temperature of the arc tube of lower than about 180.degree. C., the starting voltage increases moderately as the mercury vapor pressure increases. When the mercury vapor pressure exceeds 10.sup.1 Torr, the voltage increases sharply. When the mercury vapor pressure becomes higher than about 10.sup.2 Torr corresponding to a temperature of the arc tube of higher than 260.degree. C., the voltage abruptly increases. Under this condition, it is impossible to restart the lamp with the ordinary line voltage.
When the lamp is started at room temperature, the mercury vapor pressure in the arc tube is about 10.sup.-3 Torr. At this mercury vapor pressure, the lamp starting voltage is low and therefore the lamp can be started with the ordinary line voltage. However, in the hot restrike of the lamp, the discharge cannot be started with the ordinary line voltage because the mercury vapor pressure in the arc tube in operation is about five to fifteen atmospheres, and accordingly the starting voltage is very high. Accordingly, a certain period of time must elapse until the lamp can be restarted. That is, the restrike of the lamp cannot be carried out until the temperature of the arc tube decreases gradually, and reaches about 150.degree. C. or lower, i.e., until the mercury vapor pressure decreases to several Torr or lower.
In order to restart the discharge under the high mercury vapor pressure before the mercury vapor pressure in the arc tube decreases to several Torr or lower thereby to reduce the restriking time, it is necessary to apply a considerably high voltage to the lamp as is apparent from FIG. 3. Especially, when the mercury vapor pressure is higher than 100 Torr, it is, as a practical matter, impossible to start the discharge quickly.